Many conventional applications of therapeutic ultrasound have employed low frequency transducers. These transducers have operational frequencies that typically range from 500 kHz to 1.5 MHz. Such low frequency transducers are often preferred because they allow for acoustical energy to be focused very deep into the body, without harming the overlying tissue structures.
A conventional application of non-invasive therapeutic ultrasound using a low frequency transducer is depicted in FIG. 1. A conventional low-frequency therapeutic application 100 utilizes low frequency energy 102 to treat a deep treatment region 104, such as a deep-seated lesion. Deep treatment region 104 is located at a depth well below a superficial region of a patient. Use of the low-frequency transducer generates an isonified tissue region 106 that can range from 2 cm to 10 cm below the skin surface. Unfortunately, currently available low frequency transducers cannot be used to treat the superficial regions, thus limiting the use of low-frequency application 100. For example, most cosmetic surgeries, as well as treatment of melanomas and skin disorders, require treatment to superficial regions, thus eliminating the use of lower frequency transducers.
Another undesirable side effect of low-frequency therapy is that the acoustic energy must pass through intervening tissue layers before reaching the desired deep treatment area. The intervening layers tend to defocus the rays and absorb some of the acoustic energy. This causes the focal spot size to widen, making it difficult to control the location of the focal spot, and making dosimetry also difficult to optimize.